Method, device, and system for optical modulation in wavelength division multiplexing

ABSTRACT

An optical modulator including an optical waveguide structure and a selective modulation unit. The optical waveguide structure guides wavelength division multiplexed light including a plurality of optical carriers having different wavelengths. A modulation signal is supplied to the selective modulation unit. The selective modulation unit operates on the optical waveguide structure to selectively modulate a selected optical carrier selected from the plural optical carriers according to the modulation signal. With this configuration, the selective modulation unit operates on the single optical waveguide structure to perform selective modulation, so that optical modulation of an arbitrary optical carrier in the wavelength division multiplexed light can be easily performed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to optical modulation inwavelength division multiplexing, and more particularly to a method,device, and system for optical modulation applied to wavelength divisionmultiplexed light including a plurality of optical carriers havingdifferent wavelengths.

2. Description of the Related Art

In recent years, processing of massive amounts of information has beenneeded with development of an advanced information society, and opticalfiber communications fit for a large capacity have been applied to atransmission network for transmitting information. While a transmissionrate of information in optical fiber communications has already reached2.4 Gb/s or 10 Gb/s, a further increase in transmission capacity will beneeded in a motion picture captured communications system that isexpected to be put to practical use in the future.

Wavelength division multiplexing (WDM) is known as one of the techniquesfor increasing a transmission capacity in optical fiber communications.In WDM, wavelength division multiplexed light including a plurality ofoptical carriers having different wavelengths are used. By individuallymodulating the optical carriers, a transmission capacity per channel ofan optical fiber transmission line is increased according to the numberof WDM channels.

In the case of carrying out WDM, it is sometimes required to performoptical modulation of only an arbitrary optical carrier( e.g., arbitrarysingle optical carrier) of the plural optical carriers. For example,such optical modulation is performed in the case of transmittinginformation such as gain and S/N ratio obtained as the result ofprocessing in an optical repeater including an optical amplifier to arear-stage optical repeater by means of an arbitrary optical carrier asa supervisory signal. In such a case, a conventional method includes thesteps of spatially separating a desired optical carrier from wavelengthdivision multiplexed light by using an optical demultiplexer, nextmodulating this optical carrier, and finally combining this opticalcarrier and the other optical carriers to obtain wavelength divisionmultiplexed light again. In this manner, the conventional method has aproblem that optical modulation of an arbitrary optical carrier inwavelength division multiplexed light cannot be easily performed.Furthermore, there arises another problem that many optical elements foroptical demultiplexing and optical multiplexing are required to meet thea requirement of devices having complex configurations.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a novelmethod which can easily perform optical modulation of an arbitraryoptical carrier in wavelength division multiplexed light.

It is another object of the present invention to provide a device(optical modulator) having a simple configuration for carrying out sucha method.

It is a further object of the present invention to provide a novelsystem including such a device and suitable for construction of aflexible optical network.

In accordance with a first aspect of the present invention, there isprovided an optical modulator comprising an optical waveguide structureand selective modulation means. The optical waveguide structure guideswavelength division multiplexed light including a plurality of opticalcarriers having different wavelengths. A modulation signal is suppliedto the selective modulation means. The selective modulation meansoperates on the optical waveguide structure to selectively modulate aselected optical carrier selected from the plurality of optical carriersaccording to the modulation signal.

According to the first aspect of the present invention, the selectivemodulation means operates on the single optical waveguide structure toperform selective modulation. Accordingly, it is possible to provide anoptical modulator having a simple configuration which can easily performoptical modulation of an arbitrary optical carrier in wavelengthdivision multiplexed light.

Preferably, the selective modulation means comprises a polarizationconverter having a conversion band giving a wavelength-dependentconversion efficiency, and the conversion band includes at least onewavelength of the plurality of optical carriers. Such a wavelengthdependence of conversion efficiency in the polarization converter allowseasy selective modulation of the selected optical carrier.

To achieve substantial polarization independence in the wavelengthdivision multiplexed light, it is effective to adopt polarizationdiversity. For example, the wavelength division multiplexed light isseparated into first and second polarization components having differentpolarization planes. Each of the first and second polarizationcomponents is next subjected to selective modulation, and the first andsecond polarization components are then combined.

In accordance with a second aspect of the present invention, there isprovided a method for optical modulation applied to wavelength divisionmultiplexed light including a plurality of optical carriers havingdifferent wavelengths, comprising the steps of (a) adjusting apolarization plane of each of the plurality of optical carriers to makethe polarization plane substantially coincident with a firstpolarization plane; (b) converting the first polarization plane of aselected optical carrier selected from the plurality of optical carriersinto a second polarization plane perpendicular to the first polarizationplane according to a modulation signal; and (c) removing a polarizationcomponent having the second polarization plane.

In accordance with a third aspect of the present invention, there isprovided a method for optical modulation applied to wavelength divisionmultiplexed light including a plurality of optical carriers havingdifferent wavelengths, comprising the steps of (a) separating thewavelength division multiplexed light into a first polarizationcomponent having a first polarization plane and a second polarizationcomponent having a second polarization plane perpendicular to the firstpolarization plane; (b) supplying the first polarization component to afirst polarization converter operating according to a modulation signal;(c) removing a polarization component converted by the firstpolarization converter; (d) supplying the second polarization componentto a second polarization converter operating according to the modulationsignal; (e) removing a polarization component converted by the secondpolarization converter; and (f) combining the first and secondpolarization components after the steps (a) to (e).

In accordance with a fourth aspect of the present invention, there isprovided a system comprising an optical fiber transmission line and anoptical modulator. The optical fiber transmission line transmitswavelength division multiplexed light including a plurality of opticalcarriers having different wavelengths. The optical modulator is providedon the optical fiber transmission line, and has a configurationaccording to the first aspect of the present invention.

Preferably, the optical modulator comprises a plurality of opticalmodulators, which operate on a plurality of selected optical carriershaving different wavelengths.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a basic configuration of the opticalmodulator according to the present invention;

FIG. 2 is a block diagram showing a first preferred embodiment of theoptical modulator according to the present invention;

FIG. 3 is a view showing a preferred embodiment of the polarizationconverter applicable to the present invention;

FIGS. 4A and 4B are graphs illustrating examples of a conversion band ofthe polarization converter applicable to the present invention;

FIG. 5 is a view showing another preferred embodiment of thepolarization converter applicable to the present invention;

FIG. 6 is a plan view showing a preferred embodiment of the polarizerapplicable to the present invention;

FIG. 7 is a sectional view showing another preferred embodiment of thepolarizer applicable to the present invention;

FIG. 8 is a block diagram showing a second preferred embodiment of theoptical modulator according to the present invention;

FIG. 9 is a view showing a third preferred embodiment of the opticalmodulator according to the present invention;

FIG. 10 is a block diagram showing a preferred embodiment of the systemaccording to the present invention;

FIG. 11 is a block diagram showing another preferred embodiment of thesystem according to the present invention; and

FIG. 12 is a graph illustrating the principles of operation of thesystem shown in FIG. 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Some preferred embodiments of the present invention will now bedescribed in detail with reference to the attached drawings. Throughoutthe drawings, substantially the same parts are denoted by the samereference numerals.

FIG. 1 is a block diagram showing a basic configuration of the opticalmodulator (device for optical modulation) according to the presentinvention. This optical modulator includes an optical waveguidestructure 2 for guiding wavelength division multiplexed light (WDMlight) and a selective modulation unit 4 operating on the opticalwaveguide structure 2. The WDM light includes a plurality of opticalcarriers having different wavelengths. The selective modulation unit 4is supplied with a modulation signal to selectively modulate a selectedoptical carrier selected from the plural optical carriers according tothe modulation signal. For example, assuming that the wavelengths of theplural optical carriers are λ₁ to λ_(n) and that the wavelength of theselected optical carrier is λ₂, the selective modulation unit 4modulates the selected optical carrier only to output resultantmodulated light and passes all the optical carriers other than theselected optical carrier.

The waveforms shown in FIG. 1 correspond to the case of intensitymodulation. In this case, each optical carrier is continuous wave light(CW light), and the selected optical carrier having a wavelength λ₂ isintensity-modulated according to the modulation signal. In the case thatthis optical modulator is applied to coherent optical communication, theselective modulation unit 4 may carry out phase modulation or frequencymodulation of the selected optical carrier.

Preferably, the selective modulation unit 4 includes a polarizationconverter 6 operating according to the modulation signal and a polarizer8 operatively connected to the polarization converter 6.

In this specification, the wording that an element and another elementare operatively connected includes the case that these elements aredirectly connected, and also includes the case that these elements areso provided as to be related with each other to such an extent that anelectrical signal or an optical signal can be mutually transferredbetween these elements.

The polarization converter 6 has a conversion band giving awavelength-dependent conversion efficiency to the supplied WDM light.The polarizer 8 removes a specific polarization component of lightsupplied from the polarization converter 6. The combination of thepolarization converter 6 and the polarizer 8 allows selective opticalmodulation of the selected optical carrier. Specific configurations ofthe polarization converter 6 and the polarizer 8 will be hereinafterdescribed.

The present invention is not limited by the configuration that the WDMlight comprises optical carriers discretely arranged on a wavelengthaxis. For example, the WDM light may be white light having a broad band.In this case, continuous modulation of the WDM light is allowed bychanging the conversion band of the polarization converter 6 accordingto the modulation signal. A specific configuration of such continuousmodulation will also be hereinafter described.

FIG. 2 is a block diagram showing a first preferred embodiment of theoptical modulator. In this preferred embodiment, the selectivemodulation unit 4 is provided on an optical waveguide substrate 10. Theoptical waveguide substrate 10 has an optical waveguide 12, and theoptical waveguide structure 2 shown in FIG. 1 is provided by the opticalwaveguide 12. The optical waveguide substrate 10 is formed from a LiNbO₃substrate, for example, and the optical waveguide 12 may be fabricatedby thermal diffusion of Ti on the substrate. The polarization converter6 may be formed on the optical waveguide 12, and the polarizer 8 may beformed on or in the vicinity of the optical waveguide 12. A first end12A of the optical waveguide 12 upstream of the polarization converter 6is an input port of the selective modulation unit 4, and a second end12B of the optical waveguide 12 downstream of the polarizer 8 is anoutput port of the selective modulation unit 4.

The guided mode in the optical waveguide 12 includes two orthogonalpolarized modes, i.e., a TE mode and a TM mode. The polarization planeof TE mode light is parallel to the sheet plane of FIG. 2 and theoptical waveguide substrate 10, and the polarization plane of TM modelight is perpendicular to the sheet plane of FIG. 2 and the opticalwaveguide substrate 10. In this preferred embodiment, each opticalcarrier of the WDM light to be supplied to the input port 12A ismaintained as one of the TE mode light and the TM mode light, so as toensure stable operation of the polarization converter 6. To obtain sucha constant polarized condition of WDM light, this preferred embodimentemploys a plurality of light sources 14 (#1 to #n), a plurality ofpolarization maintaining fibers 16 (#1 to #n), and an opticalmultiplexer (MUX) 18. The light sources 14 (#1 to #n) output opticalcarriers having wavelengths λ₁ to λ_(n), respectively. These opticalcarriers are supplied through the polarization maintaining fibers 16 (#1to #n) to the optical multiplexer 18, in which the optical carriers arecombined to become WDM light. The WDM light output from the opticalmultiplexer 18 is supplied through an optical fiber 20 to the input port12A.

The operation of this optical modulator will be described based on theassumption that each optical carrier is TE mode light in the opticalwaveguide 12. The polarization converter 6 converts the TE mode lightinto TM mode light according to a modulation signal supplied. Thepolarizer 8 receives the TE mode light and the TM mode light output fromthe polarization converter 6 to remove the TM mode light and pass the TEmode light.

The polarization converter 6 has a conversion band giving awavelength-dependent conversion efficiency as mentioned above.Accordingly, by setting the conversion band so that it includes at leastone wavelength of the optical carriers, the conversion from the TE modelight of only the optical carrier having a wavelength falling in theconversion band to the TM mode light according to the modulation signalcan be carried out in the polarization converter 6. For example, in thecase that only a wavelength λ₂ is included in the conversion band of thepolarization converter 6, selective modulation of the optical carrierhaving a wavelength λ₂ can be effected. In this case, the polarizationconversion from the TE mode light to the TM mode light in thepolarization converter 6 is carried out according to the modulationsignal, so that the waveform of the modulation signal and the waveformof an optical output from the optical modulator are reverse in phase.

Referring to FIG. 3, there is shown a preferred embodiment of thepolarization converter 6. In this preferred embodiment, the polarizationconverter 6 includes an interdigital transducer (IDT) 22 formed on theoptical waveguide 12 of the optical waveguide substrate 10. The IDT 22is comprised of a pair of comb-shaped electrode patterns. The IDT 22 isconnected through terminals 24 and 26 to a control circuit 28. Thepolarization converter 6 further includes a heater 30 provided along theoptical waveguide 12. The heater 30 is connected through terminals 32and 34 to the control circuit 28. The temperature of the opticalwaveguide 12 is changed with a current flowing in the heater 30, therebychanging the refractive index of the optical waveguide 12.

A modulation signal and a selection signal are supplied to the controlcircuit 28. The control circuit 28 switches on and off a d.c. voltage(e.g., about 10 V) to be supplied to the IDT 22 according to themodulation signal supplied. For example, when the modulation signal isat a high level, the supply of the d.c. voltage to the IDT 22 goes on,and the conversion from TE mode light to TM mode light is performed.Conversely, when the modulation signal is at a low level, the supply ofthe d.c. voltage to the IDT 22 goes off, and the conversion from TE modelight to TM mode light is not performed. The control circuit 28 alsoadjusts a current flowing in the heater 30 to control the temperature ofthe optical waveguide 12.

Referring to FIG. 4A, there is shown a conversion band of thepolarization converter 6. In FIG. 4A, the vertical axis representsconversion efficiency, and the horizontal axis represents wavelength.The wavelength characteristic of conversion efficiency shown in FIG. 4Ais relatively steep. The conversion band defined by a wavelength rangegiving a conversion efficiency higher than a predetermined conversionefficiency is relatively narrow as shown by reference symbol CB1 becauseof the steep wavelength characteristic of conversion efficiency. Thecenter wavelength in the conversion band CB1 is variable according tothe temperature (refractive index) of the optical waveguide 12 shown inFIG. 3. Accordingly, an optical carrier having a wavelength included inthe conversion band CB1 can be selected by adjusting a current flowingin the heater 30 by the control circuit 28. In the example shown in FIG.4A, a wavelength λ₂ is included in the conversion band CB1, so that onlyan optical carrier having a wavelength λ₂ is subjected to conversionfrom TE mode light to TM mode light. The other optical carriers (e.g.,an optical carrier having a wavelength λ₁) are not subjected to theabove conversion, but passed through the polarization converter 6.

A desired conversion band can be obtained according to the form of theIDT 22. For example, a relatively wide conversion band CB2 as shown inFIG. 4B can be obtained by suitably designing the IDT 22. In the exampleshown in FIG. 4B, wavelengths λ₂ and λ₃ are included in the conversionband CB2, and accordingly optical carriers having wavelengths λ₂ and λ₃are subjected to conversion from TE mode light to TM mode light. It isgenerally known that the larger the number of stages in the IDT 22 (thenumber of fingers of each electrode in the IDT 22), the narrower theconversion band. Further, the conversion band can be made relativelywide or sidebands can be reduced by setting the electrode pitch of theIDT 22 according to a Gaussian distribution.

Referring to FIG. 5, there is shown another preferred embodiment of thepolarization converter 6. In this preferred embodiment, an IDT 36 isprovided on the optical waveguide 12 of the optical waveguide substrate10, so as to generate surface acoustic waves (SAW) on or in the vicinityof the surface of the optical waveguide substrate 10. The IDT 36 isconnected through terminals 38 and 40 to a control circuit 28'. Topropagate the generated SAW in a substantially one direction along theoptical waveguide 12, SAW clads 42 and 44 are provided on the oppositesides of the optical waveguide 12. Further, SAW absorbers 46 and 48 areprovided at opposite end portions of the SAW clads 42 and 44.

A modulation signal and a selection signal are supplied to the controlcircuit 28'. The control circuit 28' switches on and off an a.c. signalto be supplied to the IDT 36 according to the modulation signal, andadjusts the frequency of the a.c. signal according to the selectionsignal. When the a.c. signal is supplied to the IDT 36, an opticalcarrier having a wavelength included in a conversion band determined bythe frequency of the a.c. signal is subjected to conversion from TE modelight to TM mode light, whereas when the supply of the a.c. signal tothe IDT 36 goes off, the conversion from TE mode light to TM mode lightis not performed.

In this preferred embodiment, the conversion band or its centerwavelength is variable according to the frequency of the a.c. signal.Accordingly, a desired optical carrier can also be selectively modulatedin this preferred embodiment. For example, the center wavelength in theconversion band can be continuously changed from 1.58 μm to 1.48 μm bysweeping the frequency of the a.c. signal from 165 MHz to 180 MHz at 25°C. Alternatively, the conversion band may be adjusted by maintaining thefrequency of the a.c. signal to be supplied to the IDT 36 constant andadjusting the temperature of the optical waveguide 12 on the basis ofthe preferred embodiment shown in FIG. 3.

FIG. 6 is a plan view showing a preferred embodiment of the polarizer 8.In this preferred embodiment, the polarizer 8 consists of a pair of highrefractive index portions 50 provided along the optical waveguide 12.The optical waveguide 12 has a refractive index higher than therefractive index of the clad (undoped with Ti) of the optical waveguidesubstrate 10, and each high refractive index portion 50 has a refractiveindex equal to or higher than the refractive index of the opticalwaveguide 12. The shape parameters (e.g., length and thickness) of eachhigh refractive index portion 50 are suitably set. By suitably settingthe refractive index and the shape parameters of each high refractiveindex portion 50, only the TE mode light out of TE mode light and TMmode light guided by the optical waveguide 12 can be removed.Accordingly, in the case that this polarizer 8 is used in the preferredembodiment shown in FIG. 2, each optical carrier of the WDM light to besupplied to the input port 12A is TM mode light. The high refractiveindex portions 50 can be obtained in the same fabrication process asthat for the optical waveguide 12, so that the polarizer 8 shown in FIG.6 can be easily fabricated.

FIG. 7 is a sectional view showing another preferred embodiment of thepolarizer 8. In this preferred embodiment, the polarizer 8 includes ahigh refractive index layer 52 formed on the optical waveguide 12 and ametal layer 54 formed on the high refractive index layer 52. By suitablysetting the thickness and the refractive index of the high refractiveindex layer 52, only the TM mode light out of TE mode light and TM modelight guided by the optical waveguide 12 can be selectively removed.Alternatively, by changing the thickness and the refractive index of thehigh refractive index layer 52, the TE mode light can be selectivelyremoved. The refractive index of the high refractive index layer 52 isequal to or higher than the refractive index of the optical waveguide12.

Although not shown, the high refractive index layer 52 may be replacedby a buffer layer of Si in the sectional configuration shown in FIG. 7,thereby obtaining a polarizer for selectively removing TM mode light.

In the above preferred embodiments, the polarization converter 6converts TE mode light into TM mode light or vice versa according to amodulation signal. However, the present invention is not limited to thisconfiguration. For example, the polarization converter 6 may slightlychange a conversion efficiency according to a modulation signal, therebyperforming superimposition modulation. This configuration will now bedescribed more specifically.

FIG. 8 is a block diagram showing a second preferred embodiment of theoptical modulator. In this preferred embodiment, each of opticalcarriers to be output from light sources 14 (#1 to #n) is modulated by amain signal having a relatively high frequency (high bit rate), and thedegree of modulation of a selected optical carrier by a modulationsignal is lower than the degree of modulation by the main signal,thereby superimposing the modulation signal on the main signal. The bitrate of the main signal is 10 Gb/s, for example, and the frequency ofthe modulation signal is 10 KHz, for example. In the example shown inFIG. 8, the optical carrier having a wavelength λ₂ is a selected opticalcarrier, and the modulation signal is superimposed selectively on themain signal in the selected optical carrier. The modulation of eachoptical carrier by the main signal may be carried out by directmodulation of a laser diode or modulation by use of an externalmodulator. Such superimposition of the modulation signal on the mainsignal is effective for supervisory control in an optical repeater, forexample.

FIG. 9 is a view showing a third preferred embodiment of the opticalmodulator. In this preferred embodiment, the optical waveguide structure2 shown in FIG. 1 includes a beam splitter 56 for separating WDM lightsupplied to an input port 64 into TE mode light and TM mode light,optical waveguides 58 and 60 for propagating the TE mode light and theTM mode light, and a beam combiner 62 operatively connected to theoptical waveguides 58 and 60. Each of the beam splitter 56 and the beamcombiner 62 is provided by an X-intersection type optical waveguidestructure, and each of the optical waveguides 58 and 60 may befabricated by selective thermal diffusion of Ti into the opticalwaveguide substrate 10. An output port 66 of this optical modulator isprovided by one of the output ends of the beam combiner 62.

In this preferred embodiment, the selective modulation unit 4 shown inFIG. 1 includes a mode converter 6 (#1) and a polarizer 8 (#1) bothprovided on the optical waveguide 58, and a mode converter 6 (#2) and apolarizer 8 (#2) both provided on the optical waveguide 60. The modeconverter 6 (#1) converts the TE mode light from the beam splitter 56into TM mode light according to a modulation signal. The polarizer 8(#1) removes the TM mode light obtained by the mode converter 6 (#1).The mode converter 6 (#2) converts the TM mode light from the beamsplitter 56 into TE mode light according to the modulation signal. Thepolarizer 8 (#2) removes the TE mode light obtained by the modeconverter 6 (#2). The TE mode light and the TM mode light respectivelyoutput from the polarizers 8 (#1) and 8 (#2) are combined together bythe beam splitter 62, and resultant light is output from the output port66.

According to this configuration, optical modulation of a selectedoptical carrier can be performed separately on the polarizationcomponents having polarization planes orthogonal to each other.Therefore, there is no limitation on the polarized condition of eachoptical carrier of the WDM light to be supplied to the input port 64 inaccordance with the principle of so-called polarization diversity.

Each of the polarization converters 6 (#1) and 6 (#2) may adopt theconfiguration shown in FIG. 3 or 5. In FIG. 9, the control circuit 28 or28' is not shown. The polarizer 8 (#1) for removing TM mode light mayadopt the configuration shown in FIG. 7. The polarizer 8 (#2) forremoving TE mode light may adopt the configuration shown in FIG. 6 or 7.

FIG. 10 is a block diagram showing a preferred embodiment of the systemaccording to the present invention. This system includes terminalstations 68 and 70, an optical fiber transmission line 72 for connectingthe terminal stations 68 and 70, and a plurality of nodes 74 (#1 to #N)provided on the optical fiber transmission line 72.

Each of the nodes 74 (#1 to #N) includes an optical modulator 76according to the present invention. The terminal station 68 includes aplurality of optical senders 78 (#1 to #n) and an optical multiplexer(MUX) 80. The optical senders 78 (#1 to #n) output optical signals(optical carriers) having wavelengths λ₁ to λ_(n), respectively. Theseoptical signals are wavelength division multiplexed by the opticalmultiplexer 80, and resultant WDM signal light is output to the opticalfiber transmission line 72. Each optical modulator 76 modulates anoptical signal in a wavelength channel (e.g., wavelength λ_(i), λ_(j) orλ_(k) (i, j, and k are natural numbers not greater than n)) temporarilyor permanently given to the corresponding node 74 as a selected opticalcarrier according to a modulation signal. The terminal station 70includes an optical demultiplexer (DMUX) 82 for separating thetransmitted WDM signal light into optical signals in individual channelsand a plurality of optical receivers 84 (#1 to #n) for receiving theseparated optical signals. The optical receivers 84 (#1 to #n)regenerate transmitted data from the optical senders 78 or the opticalmodulators 76 according to the optical signals having the wavelengths λ₁to λ_(n), respectively.

According to this configuration, modulation can be sequentiallyperformed in the nodes 74 separately for the wavelength channels of theWDM signal light. That is, in the case of performing optical modulationof an arbitrary optical carrier of WDM light in a conventional system,each node must have an optical multiplexer and an optical demultiplexer,and an optical modulator must be applied to a separated arbitraryoptical carrier. To the contrary, according to the present invention, itis unnecessary to provide an optical multiplexer and an opticaldemultiplexer in each node, thereby simplifying the systemconfiguration. Further, in the system shown in FIG. 10, the selectedoptical carrier can be electrically set according to a selection signalin each optical modulator 76, thereby allowing construction of aflexible optical network. In consideration of information transmissionfrom the terminal station 68 to the terminal station 70 in the systemshown in FIG. 10, it is preferable to apply the superimpositionmodulation mentioned above with reference to FIG. 8.

Each optical modulator 76 may be designed to operate on a plurality ofselected optical carriers having different wavelengths, or two or moreoptical modulators 76 may be designed to operate on a plurality ofselected optical carriers having the same wavelength. Particularly inthe latter case, it is preferable to adopt the superimpositionmodulation mentioned above with reference to FIG. 8 and use differentmodulation signals to be superimposed in the two or more opticalmodulators 76.

Although not shown, one or plural optical amplifiers for compensatingfor attenuation of the optical signals may be provided in the opticalfiber transmission line 72.

FIG. 11 is a block diagram showing another preferred embodiment of thesystem according to the present invention. In contrast with the systemshown in FIG. 10, this preferred embodiment is characterized by amodified terminal station 68'. The terminal station 68' has a whitelight source 84 for obtaining WDM light having a continuous bandwidth.The white light source 84 includes an optical amplifier 86 foroutputting amplified spontaneous emission (ASE) and an optical band-passfilter 88 operatively connected between the optical amplifier 86 and theoptical fiber transmission line 72. The optical amplifier 86 is anerbium doped fiber amplifier (EDFA) including a doped fiber doped with arare earth element such as Er (erbium) and an optical circuit forsupplying pump light having a predetermined wavelength to the dopedfiber.

The ASE output from the optical amplifier 86 has a continuous spectrum90 as shown in FIG. 12. Accordingly, by using a relatively flat region92 of the spectrum 90, WDM light having optical power constant withrespect to wavelength can be obtained. The optical band-pass filter 88functions to extract the region 92. In this preferred embodiment, thewavelength of a selected optical carrier operated by each opticalmodulator 76 is included in the region 92.

According to the system configuration shown in FIG. 11, the modulationsignal in each optical modulator 76 can be independently demodulated inthe terminal station 70. The band to be modulated in each opticalmodulator 76 can be made sufficiently narrow in this preferredembodiment, so that the number of WDM channels can be easily increasedto thereby allow large-capacity transmission. Further, since thewavelength of a selected optical carrier in each optical modulator 76can be electrically changed, a flexible optical network system can beconstructed.

In the case that the white light source 84 as shown in FIG. 11 isadopted and that the conversion band of each optical modulator 76 issufficiently narrow as shown in FIG. 4A, the WDM light can becontinuously modulated by changing the conversion band in the region 92shown in FIG. 12, thereby allowing modulation with wavelength andconversion efficiency used as degrees of freedom and accordinglyallowing a great increase in transmission capacity.

In the preferred embodiments shown in FIGS. 3 and 5, the modulation of aselected optical carrier is made by changing the conversion efficiencyaccording to the modulation signal. In modification, however, themodulation of a selected optical carrier may be made by changing thecenter wavelength in the conversion band according to the modulationsignal.

The present invention is not limited to the details of the abovedescribed preferred embodiments. The scope of the invention is definedby the appended claims and all changes and modifications as fall withinthe equivalence of the scope of the claims are therefore to be embracedby the invention.

What is claimed is:
 1. An optical modulator comprising:an opticalwaveguide structure guiding wavelength division multiplexed lightincluding a plurality of optical carriers having different wavelengths;and selective modulation means supplied with a modulation signal tooperate on said optical waveguide structure, for selectively modulatinga selected optical carrier selected from said plurality of opticalcarriers according to said modulation signal.
 2. An optical modulatoraccording to claim 1, wherein:said optical waveguide structure comprisesa beam splitter separating said wavelength division multiplexed lightinto a first polarization component having a first polarization planeand a second polarization component having a second polarization planeperpendicular to said first polarization plane, first and second opticalwaveguides propagating said first and second polarization components,respectively, and a beam combiner operatively connected to said firstand second optical waveguides; said selective modulation means comprisesa first polarization converter provided on said first optical waveguideconverting said first polarization component into the secondpolarization component according to said modulation signal, a firstpolarizer operatively connected to said first polarization converterremoving the second polarization component, a second polarizationconverter provided on said second optical waveguide converting saidsecond polarization component into the first polarization componentaccording to said modulation signal, and a second polarizer operativelyconnected to said second polarization converter for removing the firstpolarization component; and each of said first and second polarizationconverters has a conversion band giving a wavelength dependentconversion efficiency, said conversion band including at least onewavelength of said optical carriers.
 3. An optical modulator accordingto claim 2, wherein said optical waveguide structure is provided by anoptical waveguide substrate.
 4. An optical modulator according to claim3, wherein each of said first and second polarization convertersincludes an interdigital transducer (IDT) formed on said opticalwaveguide substrate.
 5. An optical modulator according to claim 3,wherein:said first and second polarization components correspond to a TEmode and a TM mode in said optical waveguide substrate, respectively;and said first polarizer comprises a metal layer formed on said firstoptical waveguide.
 6. An optical modulator according to claim 3,wherein:said first and second polarization components correspond to a TEmode and a TM mode in said optical waveguide substrate, respectively;and said second polarizer comprises a pair of high refractive indexportions provided along said second optical waveguide.
 7. An opticalmodulator according to claim 3, wherein said optical waveguide substrateis formed of LiNbO₃.
 8. An optical modulator according to claim 1,wherein:each of said plurality of optical carriers comprises a firstpolarization component having a first polarization plane; said selectivemodulation means comprises a polarization converter supplied with saidplurality of optical carriers converting said first polarizationcomponent into a second polarization component having a secondpolarization plane perpendicular to said first polarization planeaccording to said modulation signal, and a polarizer operativelyconnected to said polarization converter removing said secondpolarization component; and said polarization converter has a conversionband giving a wavelength-dependent conversion efficiency, saidconversion band including at least one wavelength of said opticalcarriers.
 9. An optical modulator according to claim 8, wherein:saidoptical waveguide structure is provided by an optical waveguidesubstrate; and said polarization converter includes an interdigitaltransducer (IDT) formed on said optical waveguide substrate.
 10. Anoptical modulator according to claim 9, wherein:said polarizationconverter further includes means for supplying an a.c. signal to saidIDT; the conversion band of said polarization converter being adjustedaccording to the frequency of said a.c. signal.
 11. An optical modulatoraccording to claim 8, further comprising a plurality of polarizationmaintaining fibers maintaining the polarized conditions of saidplurality of optical carriers, respectively, and supplying said opticalcarriers to said polarization converter.
 12. An optical modulatoraccording to claim 9, wherein:said polarization converter furtherincludes means for supplying a d.c. signal to said IDT and means forchanging the temperature of said optical waveguide structure; theconversion band of said polarization converter being adjusted accordingto the temperature of said optical waveguide structure.
 13. An opticalmodulator according to claim 1, wherein said selective modulation meanscomprises a polarization converter having a conversion band giving awavelength-dependent conversion efficiency, a polarizer operativelyconnected to said polarization converter, and means for changing saidconversion band according to said modulation signal.
 14. An opticalmodulator according to claim 13, wherein said wavelength divisionmultiplexed light has a continuous bandwidth, and said conversion bandis changed in said bandwidth, whereby said wavelength divisionmultiplexed light is continuously modulated.
 15. An optical modulatoraccording to claim 1, wherein each of said plurality of optical carriersis modulated by a main signal, and a degree of modulation of saidselected optical carrier by said modulation signal is lower than adegree of modulation by said main signal, whereby said modulation signalis superimposed on said main signal.
 16. An optical modulator accordingto claim 1, wherein said wavelength division multiplexed light has acontinuous bandwidth.
 17. An optical modulator comprising:an opticalwaveguide structure guiding wavelength division multiplexed lightincluding a plurality of optical carriers having different wavelengths;and a modulation unit receiving a modulation signal to operate on saidoptical waveguide structure, said modulation unit selectively modulatinga selected optical carrier selected from said plurality of opticalcarriers according to the modulation signal.
 18. A system comprising:anoptical fiber transmission line for transmitting wavelength divisionmultiplexed light including a plurality of optical carriers havingdifferent wavelengths; and an optical modulator provided on said opticalfiber transmission line, said optical modulator comprising an opticalwaveguide structure guiding said wavelength division multiplexed lightand selective modulation means supplied with a modulation signal tooperate on said optical waveguide structure, for selectively modulatinga selected optical carrier selected from said plurality of opticalcarriers according to said modulation signal.
 19. A system according toclaim 18, wherein said optical modulator comprises a plurality ofoptical modulators.
 20. A system according to claim 19, wherein saidplurality of optical modulators operate on a plurality of selectedoptical carriers having different wavelengths.
 21. A system according toclaim 18, wherein said selective modulation means includes apolarization converter having a conversion band giving awavelength-dependent conversion efficiency, said conversion bandincluding at least one wavelength of said optical carriers.
 22. A methodfor optical modulation applied to wavelength division multiplexed lightincluding a plurality of optical carriers having different wavelengths,comprising:(a) adjusting a polarization plane of each of said pluralityof optical carriers to make said polarization plane substantiallycoincident with a first polarization plane; (b) converting said firstpolarization plane of a selected optical carrier selected from saidplurality of optical carriers into a second polarization planeperpendicular to said first polarization plane according to a modulationsignal; and (c) removing a polarization component having said secondpolarization plane.
 23. A method for optical modulation applied towavelength division multiplexed light including a plurality of opticalcarriers having different wavelengths, comprising:(a) separating saidwavelength division multiplexed light into a first polarizationcomponent having a first polarization plane and a second polarizationcomponent having a second polarization plane perpendicular to said firstpolarization plane; (b) supplying said first polarization component to afirst polarization converter operating according to a modulation signal;(c) removing a polarization component converted by said firstpolarization converter; (d) supplying said second polarization componentto a second polarization converter operating according to saidmodulation signal; (e) removing a polarization component converted bysaid second polarization converter; and (f) combining said first andsecond polarization components after said steps (a) to (e).